Device for igniting and energizing mercury arc rectifiers



ET AL 2,435,357

DEVICE FOR IGNITING AND ENERGIZING MERCURY ARC RECTIFIERS Feb. 3, 1948.

H. KELLER,

Filed Dec. 31 1946 Patented Feb. 3, 1948 DEVICE FOR IGNITING AND ENERGIZING MERCURY ARC RECTIFIERS Hans Keller, Wettingen, and Charles Ehrensperger, Baden, Switzerland, assignors to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application December 31, 1946, Serial No. 719,538

In Switzerland January 12, 1946 8 Claims. (Cl. 250-275) This invention relates to rectifiers and in particular to those of the vapor-electric discharge type in which rectification takes place between an anode and a pool type cathode such as mercury.

In single and multiple anode rectifiers of the class described, it is known to provide auxiliary anodes placed closer to the mercury pool than the main anode. These are commonly referred to as exciter anodes and when charged positive with respect to the mercury pool serve to maintain the rectifier in a state of ionization during the time the rectifier apparatus is in operation. When the auxiliary anodes are fed from an alternating current source, at least two anodes are required, and when fed with direct current, at least one anode is necessary.

It is also common to use a device such as an immersion type igniter or starter electrode in conjunction with the auxiliary anodes for initially establishing a cathode spot to ionize the mercury, from which a sustaining discharge to the auxiliary anodes then takes place. Thestarter electrode is removed from operation as soon as the discharge is taken up by the exciter anodes and remains out of service as long as the rectifier remains ionized. However, should for any reason the apparatu become deionized, the starter electrode is automatically reenergized to reestablish a cathode spot that is again transferred to the eXciter anodes.

It is also known to control pool type rectifiers using a single main anode by means of a resistance igniter electrode in contact with the mercury to periodically reestablish a cathode spot for each cycle of the alternating current power source that supplies the main anode, such periodic operation being attained with the aid of a voltage impulse applied to the igniter electrode in timed relation with the alternating current voltcharge between the pool and the exciter anode,

age applied between the main anode and Iner- V cury pool.

Mercury arc rectifier operation in accordance with the first of the two methods outlined above, especially those with but a single main anode, has the disadvantage that the sustaining are between the exciter anodes and the mercury pool is extinguished much too frequently because of the wide fluctuations in potential of the main anode and this of course necessitates an altogether too frequent use of the starter electrode with its attendant high current characteristic.

Operation of the rectifier in accordance with the second method has the drawback in that a comparatively high value of control current is and automatic changeover of the starting current form the immersion starter electrode to the exciter anode as soon as the latter has picked up the arc.

A more specific object is to provide a novel and improved auxiliary arc starting and sustaining device for pool rectifiers comprising an inverted cup shaped exciter anode located closely adjacent the pool surface, and an arc igniter electrode imbedded in the anode, the latter depending from the anode and dipping into the pool cathode.

The foregoing and other objects and advantages to be derived from the invention will become more apparent from the following detailed description of a preferred construction and from the accompanying drawing.

In the drawings, Fig. 1 is substantially a vertical central section through the combined exciter anode and resistance igniter structure with certain of the parts shown in elevation, and Fig. 2 is a plot showing the characteristic of the operating voltage and current requirements of the device.

The mercury arc rectifier comprises an evacuated metal tank having a bottom wall I and which contains a pool of mercury 2. The remainder of the tank, as well as the main anode from which power. rectification takes place, have not been shown since these are of standard and well known construction and bear only an indirect relation to the features of the present invention.

The igniter electrode 4 is of the immersion resistance type and consists of a body of high resistance composition such as boron carbide or silicon carbide fashioned. in the form of a short cylinder, the lower end terminating in a long within an opening through the bottom wall of v= the tank. The lower face of anoderests upon the upper end face of bushing 8 aJnd-the-latter is hermetically sealed to both the stud and tank to maintain a good vacuumwithin-the tank. Stud, 3 also functions as a" lead-through con ductor between anode 5 and" connectionsm'ad'e outside the tank, and its lower outside end is accordingly fashioned into a terminall J The manner of operation of the deviceis as follows:

With reference now to Fig. 2, application of a' sufficientlyhigh ignition voltage ezof short duration to the combined exciter'anode and igniter electrode structure viathe conductive stud '3 will first establish a cathode spot at the place where the tipof the igniter electrode 4! clips intothe mercury poo'l cathodei; As soon as this spot' has been established, the attendant ionization of the mercury causes a sustaining arc to be established between thev rim 6 of the exciter anode and the spot in pool When this" has occurred,'only a fraction of the initially'highstarting current will still flow-between pool 2 and igniter electrode 4 because of the latters high' ohmic resistance. The are .favor the lower edge of the rim 6 of theaexciter anode 5 as aplace of attachment since this part of the anode is located closely adjacent the surface of pool 2 and-aver small current is therefore-all that is required to maintainthe arc, as distinguished from the relatively much higher valueof current :initial-ly necessary to first establish-the cathode. spot on the mercury-pool When starting the rectifier. The curve of the ignition and exciter current-is shown in Fig. 2. plotted against time. At time ti a high-peaked voltage is impressed on the igniterelectrode and the. ensuing highwcurrent istarts the-cathode spot, while at moment t2 after the ignition has been accomplished, a much'smaller value of current i1 is sufficient for maintaining the exciting are between the anode 5 and pool'2.

After I the arc. has been-transferred 'from the igniter electrode t'to exciter anode '5; insulating cap '1. prevents'ithe are from climbing rim 6.

The improved construction, structurally combining the ignition electrode with the exciter anode hasthe advantage in that it is relatively insen'sitive to potential fluctuations of the anode since the ignition and exciting device-is practically-at thelpotential of the pool cathode. Further the ign'iter element 4 normally operates only when starting'the rectifier and hence the initially high ignition current is inconsequential.

The'impr oved ignite'r and exciter device accordingto the invention can be used both for single and multiple' anode types of pool type rectifiers.

We claim-: I a

1. A device for igniting and maintaining a cathode spot in a pool cathode type of rectifier, said device comprising, an exciter anode supported closely adjacent the surface of the pool cathode, and a high resistance igniter electrode depending from said exciter anode. and dipping into the pool cathode.

2. A device for igniting and maintaining a cathode spot in a pool cathode type of rectifier, saiddevice comprising, an inverted cupped exciter anode-"supported with the rim thereof closely adjacent the surface of the pool cathode, and an igniter electrode of high resistance material extending downwardly from said exciter anode within'therimm'ed portion and dipping into the pool cathode.

3. A device forstartin'g rectifiers as defined in claim. 2 Wherein electrical connection thereto is made through a single insulated conductorcarriedthrough the rectifier tank and cathode pool, and said conductor serves simultaneously as a current lead-into both the igniter'electrode and exciter anode;

4. A device forstarting rectifiers as definedin claim 2 wherein electrical connection thereto is made through a single rigid insulated conductor carried through the wall of the rectifier tank and the cathode pool, said' conductor functioning as a support for said exciter anode above the pool surface andas a current lead-in to both said anode and igniter electrode;

5. A'device for starting rectifiers as defined in claim 2 wherein said exciter anode is made of graphite.

6. A device for starting rectifiers as defined in claim 2 and further includinga cap of insulating material covering all of the exterior surface of said exciter anode with the exception of the rim portion.

'7. A device for igniting and maintaining a cathode spot in a pool cathode'type of rectifier, said device comprising, an inverted cupped exciter anode supported with the rim thereof closely adjaeentthe surface of the pool cathode, and an 'igniter electrode imbedded in the body portion of said anode, said electrode being of high resistance material and dependingfromwithin the. rimmed portion of said anode into the pool'cathode. 8. A device-for igniting and maintaining a cathode spot in a pool cathode type of rectifier, said device comprising, an inverted cup shaped exciter anode having an annular rim, a terminal stud extending through an insulated opening in .the bottom wall of the rectifier tank and through the cathode pool for supporting said exciter anode with the rim portion thereofclosely adjacent the surface of the pool, an insulating member capping said anode above the rim portion, and an igniter electrode imbe'dded within the body of said anode and surrounded by the rim portion, said. electrode having a high electrical resistance characteristic and dipping into the cathode pool.

HANS KELLER. CHARLES EHRENSPERGER. 

